The invention relates to a two-wire extended serial bus communication system provided with a synchronizing mechanism. In real-time applications of the well-known VME-bus organization, an extensive and versatile I/O system is essential. The maximum configuration of such a system should be capable of handling more inputs and outputs than can be fitted into one VME-bus rack. The powerful functionally and high performance of the VME-bus interface logic and backplane usually are not needed by the I/O system, and hence much of the cost and complexity of these provisions is wasted. When a large number of inputs and outputs is desired, a serial organization would be preferred.